Solar cell, solar cell module, and method for manufacturing solar cell

ABSTRACT

A solar cell has a collecting electrode formed therein. The collecting electrode is provided with: a main conductive layer that contains copper; and an overcoat layer that covers at least a part of the main conductive layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation under 35 U.S.C. §120 ofPCT/JP2011/072325, filed Sep. 29, 2011, which is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a solar cell, a solar cell module, anda manufacturing method of a solar cell.

BACKGROUND ART

In recent years, solar cells which can convert solar light intoelectrical energy have been used as an alternative energy source forpetroleum. Solar cells include a monocrystalline solar cell, apolycrystalline solar cell, an amorphous solar cell, or a combination ofthese solar cells.

In a solar cell, in order to extract the generated electric power to theoutside, a collecting electrode such as a finger portion and a bus barportion is provided on a front surface and a back surface of the solarcell. In recent years, in consideration of the high conductivity, easeof machining, and price of material, a collecting electrode formed byplating is often employed, as described in Patent Document 1. When thecollecting electrode is formed by plating, in the related art, materialssuch as nickel (Ni), copper (Cu),chrome (Cr), zinc (Zn), tin (Sn), andsilver (Ag) are used. In particular, in relation to the cost andresistivity, copper is preferably used.

RELATED ART REFERENCES Patent Document [Patent Document 1] JP 2000-58885A DISCLOSURE OF INVENTION Technical Problem

A solar cell is in many cases used in a form of a solar cell module inwhich the solar cell is sealed in a sealing layer between aencapsulating member on a side of a light receiving surface and aencapsulating member on a side of a back surface. For solar cell modulesnormally used outdoors, in addition to the output characteristic,reliability is also required.

Solution to Problem

According to one aspect of the present invention, there is provided asolar cell in which a collecting electrode is formed on at least one ofa light receiving surface and a back surface, wherein the collectingelectrode comprises a primary conductive layer including copper, and atleast one of an overcoat layer covering an upper surface side of theprimary conductive layer and an undercoat layer covering a lower surfaceside of the primary conductive layer.

According to another aspect of the present invention, there is provideda method of manufacturing a solar cell, comprising: a first step offorming a coating layer having an opening over at least one of a lightreceiving surface and a back surface; a second step of plating the onesurface which is exposed in the opening with a metal, to thereby formacollecting electrode having a lower thickness than that of the coatinglayer; and a third step of covering the coating layer and the collectingelectrode with a sealing layer, wherein the second step comprises a stepof forming a primary conductive layer including copper, and a step offorming at least one of an overcoat layer covering an upper surface sideof the primary conductive layer and an undercoat layer covering a lowersurface side of the primary conductive layer.

Advantageous Effects of Invention

According to various aspects of the present invention, reliability of asolar cell module can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional diagram showing a structure of a solar cellmodule according to a first preferred embodiment of the presentinvention.

FIG. 2 is a plan view showing a structure of the solar cell moduleaccording to the first preferred embodiment of the present invention.

FIG. 3 is an enlarged cross sectional diagram showing a structure of thesolar cell module according to the first preferred embodiment of thepresent invention.

FIG. 4 is an enlarged cross sectional diagram showing anotherconfiguration of the structure of the solar cell module according to thefirst preferred embodiment of the present invention.

FIG. 5 is a cross sectional diagram showing a structure of a solar cellmodule according to a second preferred embodiment of the presentinvention.

FIG. 6 is an enlarged cross sectional diagram showing a structure of thesolar cell according to the second preferred embodiment of the presentinvention.

FIG. 7 is an enlarged cross sectional diagram showing anotherconfiguration of the structure of the solar cell module according to thesecond preferred embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION First Preferred Embodiment

As shown in a cross sectional diagram of FIG. 1, a solar cell 100according to a first preferred embodiment of the present inventioncomprises a substrate 10, an i-type amorphous layer 12 i, a p-typeamorphous layer 12 p, a transparent conductive layer 14, an i-typeamorphous layer 16 i, an n-type amorphous layer 16 n, a transparentconductive layer 18, coating layers 20 and 22, and collecting electrodes24 and 26. The i-type amorphous layer 12 i, the p-type amorphous layer12 p, the i-type amorphous layer 16 i, and the n-type amorphous layer 16n may include crystal grains. In addition, a solar cell module 300comprises the solar cell 100, sealing layers 28 and 30, andencapsulating members 32 and 34.

A structure of the solar cell 100 will now be described while amanufacturing method of the solar cell 100 is described.

The substrate 10 is a wafer-shaped plate made of a crystallinesemiconductor material. The substrate 10 may be a substrate made of acrystalline semiconductor of an n-type conductivity or a p-typeconductivity. For the substrate 10, for example, a monocrystallinesilicon substrate, a polycrystalline silicon substrate, a galliumarsenide substrate (GaAs), an indium phosphide substrate (InP), or thelike may be employed. The substrate 10 absorbs incident light andproduces carrier pairs of electrons and holes by a photoelectricconversion effect. In the following, an example configuration will bedescribed which uses a substrate made of n-type monocrystalline siliconas the substrate 10.

First, a pre-process such as cleaning is applied on the substrate 10. Inthe pre-process, a projection-and-depression structure which is called atextured structure is formed on at least the light receiving surface ofthe substrate 10.

The i-type amorphous layer 12 i is, for example, an intrinsic amorphoussilicon semiconductor layer formed over one primary surface of thesubstrate 10. The i-type amorphous layer 12 i may contain hydrogen. Thep-type amorphous layer 12 p is formed over the i-type amorphous layer 12i. The p-type amorphous layer 12 p is, for example, a p-type amorphoussilicon semiconductor layer. The p-type amorphous layer 12 p may containhydrogen.

The i-type amorphous layer 12 i is inserted between the p-type amorphouslayer 12 p and the substrate 10 in order to improve a contactcharacteristic between the p-type amorphous layer 12 p and the substrate10. Because of this, a thickness of the i-type amorphous layer 12 i isset to a thickness which would substantially not contribute to powergeneration, for example, in a range of greater than or equal to 0.1 nmand less than or equal to 25 nm, and more preferably, in a range ofgreater than or equal to 5 nm and less than or equal to 10 nm.

The i-type amorphous layer 12 i and the p-type amorphous layer 12 p maybe formed through film formation methods such as CVD, such as plasmachemical vapor deposition or sputtering.

The i-type amorphous layer 16 i is, for example, an intrinsic amorphoussilicon semiconductor layer formed over the other primary surface of thesubstrate 10. The i-type amorphous layer 16 i may contain hydrogen. Then-type amorphous layer 16 n is formed over the i-type amorphous layer 16i. For example, the n-type amorphous layer 16 n is an n-type amorphoussilicon semiconductor layer. The n-type amorphous layer 16 n may containhydrogen.

The i-type amorphous layer 16 i is inserted between the n-type amorphouslayer 16 n and the substrate 10 in order to improve a contactcharacteristic between the n-type amorphous layer 16 n and the substrate10. Because of this, similar to the i-type amorphous layer 12 i, athickness of the i-type amorphous layer 16 i is set to a thickness whichwould substantially not contribute to the power generation, for example,in a range of greater than or equal to 0.1 nm and less than or equal to25 nm, more preferably, in a range of greater than or equal to 5 nm andless than or equal to 10 nm.

The i-type amorphous layer 16 i and the n-type amorphous layer 16 n canbe formed through film formation methods such as CVD, sputtering, or thelike.

The transparent conductive layer 14 is formed over the p-type amorphouslayer 12 p, and the transparent conductive layer 18 is formed over then-type amorphous layer 16 n. The transparent conductive layers 14 and 18are formed including, for example, at least one of metal oxides such asindium oxide, zinc oxide, tin oxide, and titanium oxide, and these metaloxides may be doped with a dopant such as tin, zinc, tungsten, antimony,titanium, cerium, and gallium. The transparent conductive layers 14 and18 may be formed though film formation methods such as evaporation, CVD,and sputtering. Thicknesses of the transparent conductive layers 14 and18 can be suitably adjusted by indices of refraction of the transparentconductive layers 14 and 18, or the like.

The coating layer 20 is formed over the transparent conductive layer 14,and the coating layer 22 is formed over the transparent conductive layer18. The coating layers 20 and 22 are preferably insulating because thecollecting electrodes 24 and 26 will be formed through electroplating aswill be described later, but the structure of the coating layers 20 and22 is not limited to such a structure. The coating layers 20 and 22 arepreferably made of a photo-curing resin in consideration of themachinability. For example, the coating layers 20 and 22 are made of anepoxy-based, an acrylate-based, or an olefin-based ultraviolet curingresin.

The coating layers 20 and 22 can be formed over the transparentconductive layers 14 and 18 through spin-coating, spraying, or printing.Thicknesses of the coating layers 20 and 22 can be adjusted, when thecoating layers 20 and 22 are formed using resin materials, by theviscosity of the resin material or the formation conditions. Forexample, when spin-coating is employed, the thickness can be adjusted bythe conditions such as the rotational speed and number of rotations ofthe spin-coat. As shown in a plan view of FIG. 2, the coating layers 20and 22 are formed in a pattern having an opening A (refer to FIG. 1)corresponding to the shapes of the collecting electrodes 24 and 26 to bedescribed later. With the use of the photo-curing resin for the coatinglayers 20 and 22, the coating layers 20 and 22 can be patterned byapplying a photolithography technique. The opening A is formed such thata part of the transparent conductive layers 14 and 18, which are layersbelow the coating layers 20 and 22, is exposed.

The collecting electrode 24 is a conductive layer over the transparentconductive layer 14, and the collecting electrode 26 is a conductivelayer over the transparent conductive layer 18. The collecting layers 24and 26 preferably have an comb-shaped structure including a plurality offinger portions and a bus bar portion connecting the plurality of fingerportions, so that the electric power generated by the solar cell 100 canbe evenly collected.

The collecting electrode will now be descried exemplifying thecollecting electrode 24. As shown in an enlarged cross sectional diagramof FIG. 3, the collecting electrode 24 has a multilayer structureincluding a primary conductive layer 24 a and an overcoat layer 24 b.The primary conductive layer 24 a is a metal layer having a primarycomposition of copper, in consideration of the high conductivity, theease of machining, and the price of the material. The overcoat layer 24b is an overcoating layer of the primary conductive layer 24 a, and isplaced between the primary conductive layer 24 a and the sealing layer28 to be described later. The overcoat layer 24 b is a metal layerincluding at least one of nickel (Ni), tin (Sn), titanium (Ti), tantalum(Ta), tungsten (W), and palladium (Pd). It is sufficient that theovercoat layer 24 b cover at least a part of the primary conductivelayer 24 a so that the primary conductive layer 24 a does not directlycontact the sealing layer 28, but the overcoat layer 24 b preferablycovers the entire primary conductive layer 24 a.

A thickness of the primary conductive layer 24 a may be determinedaccording to a width of the collecting electrode 24 and necessaryconductivity, and is preferably set, for example, to about a few tens ofμm. A thickness of the overcoat layer 24 b is preferably set in a rangeof greater than or equal to 0.2 μm and less than or equal to 10 μmaccording to a diffusion characteristic of copper among the materialsforming the overcoat layer 24 b. For example, because nickel has asmaller diffusion coefficient with respect to copper, when the overcoatlayer 24 b is formed using nickel, the thickness may be set to begreater than or equal to 0.2 μm, and when the overcoat layer 24 b isformed with tin, the thickness is preferably set to be greater than orequal to 5 μm.

The collecting electrode 24 is embedded in the opening A of the coatinglayer 20. A thickness of the collecting electrode 24 may be set thinnerthan the thickness of the coating layer 20 as shown in FIG. 3, or may beset thicker than the thickness of the coating layer 20 as shown in FIG.4. By setting the thickness of the collecting electrode 24 to be thinnerthan the thickness of the coating layer 20, a depression formed by anupper surface of the collecting electrode 24 and side surfaces of thecoating layer 20 is formed in the opening A. The sealing layer 28 thenenters the depression of the opening A. Because of this, even when thesealing layer 28 is thermally expanded during the use, the thermalexpansion of the sealing layer 28 is inhibited by the side surfaces ofthe opening A of the coating layer 20. As a result, a close-contactproperty of the coating layer 20 and the sealing layer 28 can beimproved, and the detachment of the coating layer 20 and the sealinglayer 28 can be inhibited.

In addition, a contact area between the collecting electrode 24 and thesealing layer 28 is reduced, and thus the reliability of the solar cellmodule 300 can be improved as will be described later. On the otherhand, by setting the thickness of the collecting electrode 24 to bethicker than the thickness of the coating layer 20, it becomes possibleto obtain necessary conductivity for the collecting electrode 24 evenwhen the width of the collecting electrode 24 is narrowed, andconsequently, an advantage can be obtained in that an effective area oflight reception can be increased, or the like.

The formation method of the collecting electrode 24 is not particularlylimited, but the collecting electrode 24 is preferably formed throughelectroplating. After the insulating coating layer 20 having the openingA is formed, a voltage is applied on the transparent conductive layer 14and the collecting electrode 24 is formed through electroplating. Afterthe primary conductive layer 24 a is formed, the overcoat layer 24 b isformed over the primary conductive layer 24 a. Thicknesses of theprimary conductive layer 24 a and the overcoat layer 24 b can beadjusted by conditions such as an applied voltage, a current value, afilm formation duration, etc., during the electroplating.

The thicknesses of the coating layer 20 and the collecting electrode 24can be checked by a cross-sectional observation using an electronmicroscope. For example, a relationship between the thicknesses of thecoating layer 20 and the collecting electrode 24 can be identified usinga cross sectional SEM or a cross sectional TEM.

The solar cell 100 is sealed by the encapsulating member 32. The sealinglayer 28 is positioned over the coating layer 20 and the collectingelectrode 24, and the encapsulating member 32 is pressed under a heatedcondition to seal the solar cell 100. The sealing layer 28 is preferablya resin such as ethylene vinyl acetate (EVA), polyvinyl butyrate (PVB).The encapsulating member on the side of the front surface (lightreceiving surface) of the solar cell 100 is preferably a glass plate ora resin sheet having a light transmitting characteristic. For theencapsulating member on the side of the back surface of the solar cell100, in addition to the glass plate and resin sheet having lighttransmitting characteristic, a light blocking member such as a resinfilm formed by sandwiching a metal foil may be employed. Although notshown in the figures, the solar cell module 300 includes a plurality ofsolar cells 100 electrically connected by wiring members. The wiringmember is generally connected to the bus bar portion of the collectingelectrode.

In this manner, with the overcoat layer 24 b covering the primaryconductive layer 24 a, diffusion of copper contained in the primaryconductive layer 24 a into the sealing layer 28 is inhibited, and colorchange and degradation of the sealing layer 28 can be prevented. Or,with the overcoat layer 24 b, oxidation of copper contained in theprimary conductive layer 24 a is inhibited, and color change anddegradation of the primary conductive layer 24 a can be prevented. As aresult of these measures, reliability of the solar cell module 300 canbe improved.

In the above description, the coating layer 20, the collecting electrode24, the sealing layer 28, and the encapsulating member 32 are described.The coating layer 22, the collecting electrode 26, the sealing layer 30,and the encapsulating member 34 on the opposite side may have similarstructures. However, when the above-described structure is employed inat least a part of one of the sides, the advantage can be obtained to acertain degree.

Second Preferred Embodiment

As shown in a cross sectional diagram of FIG. 5, a solar cell 200according to a second preferred embodiment of the present inventioncomprises the substrate 10, the i-type amorphous layer 12 i, the p-typeamorphous layer 12 p, the transparent conductive layer 14, the i-typeamorphous layer 16 i, the n-type amorphous layer 16 n, the transparentconductive layer 18, the coating layers 20 and 22, and collectingelectrodes 36 and 38. A solar cell module 400 includes the solar cell200, the sealing layers 28 and 30, and the encapsulating members 32 and34.

The solar cell 200 has a similar structure to that of the solar cell 100in the first preferred embodiment of the present invention except thatthe structures of the collecting electrodes 34 and 36 differ from thosein the first preferred embodiment. Therefore, in the following, thecollecting electrodes 34 and 36 will be described and other structureswill not be described again.

In the following, the collecting electrodes will be describedexemplifying the collecting electrode 36. As shown in an enlarged crosssectional diagram of FIG. 6, the collecting electrode 36 has amultilayer structure of an undercoat layer 36 a and a primary conductivelayer 36 b. The collecting electrode 36 is embedded in an opening B ofthe coating layer 20. The undercoat layer 36 a is an undercoating layerof the primary conductive layer 36 b, and is provided between thetransparent conductive layer 14 and the primary conductive layer 36 b sothat the primary conductive layer 36 b does not directly contact thetransparent conductive layer 14. The primary conductive layer 36 b isprovided over the undercoat layer 36 a. The undercoat layer 36 a is ametal layer including at least one of nickel, tin, titanium, tantalum,tungsten, and palladium. The primary conductive layer 36 b is a metallayer having a primary composition of copper (Cu). A thickness of theundercoat layer 36 a may be preferably set in a range of greater than orequal to 0.2 μm and less than or equal to 10 μm, according to thematerials forming the undercoat layer 36 a. For example, because nickelhas a smaller diffusion coefficient with respect to copper, when theundercoat layer 36 a is formed with nickel, the thickness may be set tobe greater than or equal to 0.2 μm, and when the undercoat layer 36 a isformed with tin, the thickness is preferably set to be greater than orequal to 5 μm. A thickness of the primary conductive layer 36 b ispreferably set to be about a few tens of μm. A thickness of thecollecting electrode 36 may be set to be thinner than the thickness ofthe coating layer 20 as shown in FIG. 6 or thicker than the thickness ofthe coating layer 20 as shown in FIG. 7.

The collecting electrode 36 is preferably formed through electroplating.After an insulating coating layer 20 having an opening B is formed, avoltage is applied to the transparent conductive layer 14, and theundercoat layer 36 a is formed through electroplating. Then, the primaryconductive layer 36 b is formed over the undercoat layer 36 a. Thethicknesses of the undercoat layer 36 a and the primary conductive layer36 b can be adjusted by conditions such as an applied voltage, a currentvalue, and a film formation duration, etc., during the electroplating.

In this manner, by providing the undercoat layer 36 a between thesemiconductor which is a photoelectric conversion region and the primaryconductive layer 36 b, diffusion of copper contained in the primaryconductive layer 36 b to the semiconductor can be inhibited, andreduction in the power generation characteristic can be inhibited. As aresult, reliability of the solar cell module 400 can be improved. In theabove description, the collecting electrode 36 has been described. Thecollecting electrode 38 on the opposite side may be similarly formed.However, when the above-described structure is employed in at least apart of one of the sides, the advantage can be obtained to a certaindegree.

Alternatively, the structure according to the first preferred embodimentof the present invention and the structure according to the secondpreferred embodiment of the present invention may be combined. In otherwords, the collecting electrode may be formed in a layered structure ofthe undercoat layer, primary conductive layer, and overcoat layer. Inaddition, the overcoat layer or the undercoat layer may have a layeredstructure of a plurality of materials.

Moreover, for the overcoat layer or the undercoat layer, an epoxy-basedresin or the like may be employed. In this case, the overcoat layer orthe undercoat layer may be formed by applying the resin through screenprinting, inkjet printing, or the like. In addition, when a resin isused for the overcoat layer or the undercoat layer, an opening forelectrically connecting the primary conductive layer and otherconductive members may become necessary in the overcoat layer or theundercoat layer.

Moreover, the application of the present invention is not limited to acrystalline solar cell, and the present invention may be similarlyapplied to other types of thin film solar cells so long as the solarcell has the collecting electrode.

1. A solar cell in which a collecting electrode is formed on at leastone of a light receiving surface and a back surface, wherein thecollecting electrode comprises a primary conductive layer includingcopper, and at least one of an overcoat layer covering an upper surfaceside of the primary conductive layer and an undercoat layer covering alower surface side of the primary conductive layer.
 2. The solar cellaccording to claim 1, wherein the overcoat layer or the undercoat layeris a metal layer including at least one of nickel, tin, titanium,tantalum, tungsten, and palladium.
 3. The solar cell according to claim1, wherein the overcoat layer or the undercoat layer is a resin layer.4. The solar cell according to claim 1, wherein the collecting electrodeis formed between insulating coating layers, and a thickness of thecollecting electrode is lower than a thickness of the coating layer. 5.The solar cell according to claim 2, wherein the collecting electrode isformed between insulating coating layers, and a thickness of thecollecting electrode is lower than a thickness of the coating layer. 6.The solar cell according to claim 3, wherein the collecting electrode isformed between insulating coating layers, and a thickness of thecollecting electrode is lower than a thickness of the coating layer. 7.A solar cell module having the solar cell according to claim 1,comprising a sealing layer of a resin covering an upper surface side ofthe collecting electrode.
 8. A solar cell module having the solar cellaccording to claim 2, comprising a sealing layer of a resin covering anupper surface side of the collecting electrode.
 9. A solar cell modulehaving the solar cell according to claim 3, comprising a sealing layerof a resin covering an upper surface side of the collecting electrode.10. A solar cell module having the solar cell according to claim 4,comprising a sealing layer of a resin covering an upper surface side ofthe collecting electrode.